Studies on the immunologic reactivity to poison oak and ivy in humans indicate that the reaction to this oil product of the plant is mediated by T lymphocytes, with a requirement for macrophages. In vitro studies indicate that individuals have reactive cells in their peripheral blood from 2-10 weeks after the dermatitis. The compound acts in two ways. First, it probably binds to surface proteins on the host cells, and the host then reacts to the altered surface proteins. The methods of binding is unclear, but there is a requirement for a catechol ring, which suggests that a covalent bond may be formed. Additionally however the molecule can concentrate into the cell membranes and alter the fluidity of the cell membranes. This effect apparently has an effect on the presentation of the normal membrane bound proteins to the immune system, and may make them more antigenic.